Like "Full image not aligned properly in div" troubles with other bits of NextGen.
Tried putting different values into the ng-gallery.css (ngg-imagebrowser-nav/back/next) to force it to move but that didn't help. Tried sticking it into a table inside the div with fixed sizes to see where it was picking up the right shift from, no clues. Basically, looks like it's something inside NextGen itself (when the image is being drawn?) that I'm too busy to pick apart. The html output seems fine.
Worked before, doesn't work now.
Would roll back if I had time.
Current versions of WP 2.9.2 and NextGen 1.5.3 as of 04/30/10